PROJECT SUMMARY/ABSTRACT This application investigates the relationship between mitochondrial apoptotic stress, senescence and aging. Cellular senescence is a well-established driver of tissue and organismal aging, a process thought to be partly mediated via the induction of a chronic Senescence-associated secretory phenotype (SASP). Consequently, there is great interest in selectively targeting senescent cells as a strategy to promote healthy aging. Our work has demonstrated that mitochondrial dysfunction is a hallmark of cellular senescence and a driver of the SASP. Mitochondria are also major regulators of apoptosis, a process which involves mitochondrial outer membrane permeability (MOMP) and subsequent release of cytochrome c through the actions of the pro-apoptotic proteins BAX and BAK. We found that MOMP occurring in a small subset of mitochondria without inducing cell-death, a process known as minority MOMP (miMOMP) is a feature of cellular senescence and contributes to the SASP. We have also observed that miMOMP leads to the release of cytosolic mtDNA which can engage the cGAS/STING pathway, a major regulator of the SASP. These data led us to hypothesize that miMOMP is a major contributor to the senescent phenotype and may be a novel target for interventions aiming to counteract aging and age-related pathology. We will conduct in vitro experiments in which we will explore the molecular mechanisms of why miMOMP occurs during senescence and how it impacts on senescence and the SASP (aims 1 and 2). Finally, we will investigate the impact of genetic and pharmacologic interventions which alleviate miMOMP during aging in vivo (aim 3). For that, we will unravel the relative impact of conditional deletion of BAX/BAK (which drive miMOMP) and/or Apaf1 (essential for MOMP- dependent caspase activation) during aging in vivo. Finally, we will test the effectiveness of miMOMP-inhibiting drugs on improving key phenotypes in aging mice. Our ultimate goal is to identify new interventions that target senescent cells to alleviate age- related dysfunction.